Fitting the control knob

I recently fitted a replacement contol knob to a Sirrus 1600 showers but the water temperature doesn't get as hot as it did before. When I remove the control knob and also the small circlip from the serrated part (spindle for want of a better description) that protrudes from the cartridge, I can turn the spindle by hand to allow the water to flow; when doing so, I notice that the spindle retracts into the cartridge further than it would be able to with the circlip fitted and the water is piping hot and the brass becomes hot to touch. When I wind the spindle back out to refit the circlip and the contol knob, I can't achieve the same hot temperature. What am I doing wrong? There are two grooves to accept the circlip on the spindle; should it be fitted on closest groove to the cartridge or the one closer to the front of the spindle? What position should the spindle be in, further in or out of the cartridge, when I put the control know back on and refit the screw in the centre? Your help will be appreciated - cold showers - grrr!